BALTIMORE -- One recent afternoon, when Harold Moore and Clarence Hall descended on a dreary scrap of asphalt that serves as a thriving open air drug market, they were welcomed by a dozen red-eyed men eager for the small, wrapped packages they carried.

"Need some condoms?" asked Moore, pulling wrapped prophylactics from a red duffel bag and handing them to his willing customers. "How about some bleach?"

Every day, public health officials here estimate, four intravenous drug users contract the human immunodeficiency virus that causes AIDS. Most don't know it and continue to share needles and have sex without using condoms, unwittingly infecting their wives or girlfriends and unborn babies.

Moore and Hall, street outreach workers for the Health Education and Resource Organization (HERO), this city's oldest AIDS organization, are trying to slow that rate by convincing intravenous drug users to avoid sharing needles or, if they must, to first flush them with bleach and water, which kills the virus. Every weekday afternoon, the two men dispense free condoms and pamphlets on AIDS prevention and try to persuade addicts to enter drug treatment programs.

Here, as in many cities around the country, IV drug users are the fastest-growing group of AIDS patients. Nearly half of the 2,521 cases of AIDS reported in Maryland since 1981 have been from this city of 750,000.

Drug users currently account for 35 percent of Baltimore's AIDS cases, a figure higher than the national rate of 28 percent reported by the Centers for Disease Control.

Many drug users do not know they are infected, according to Gabor Kelen, an emergency room physician at Johns Hopkins Hospital. In 1987, Kelen told a congressional committee, 19 percent of intravenous drug users treated in the hospital's emergency room tested positive; in 1989, 34 percent of those admitted to the emergency room had undiagnosed HIV infection.

"We're starting to see more and more people who were infected because of heterosexual sex with an IV drug user," he said.

To slow the spread of AIDS among the city's estimated 55,000 addicts, many of whom shoot heroin, city officials are considering a clean needle program, similar to those under way in several U.S. cities and in Europe.

Supporters of such programs, recently started in Tacoma, Wash.; Portland, Ore.; Boulder, Colo., and Vancouver, British Columbia, say they are effective because many addicts desperately want to avoid contracting AIDS. But such programs have proved to be enormously controversial because, critics charge, they encourage drug use. A pilot needle exchange program in New York was killed shortly after that city's new mayor, David N. Dinkins, took office.

Last month, health officials here met with the Baltimore City Council to propose a pilot program for 100 addicts. Maxie T. Collier, the city's health commissioner who supports a needle exchange, said the city has applied for $300,000 in private funds to launch the project.

James Shellington, director of HERO's six-member outreach program, said he regards needle exchange as a necessary evil. "The reality is, some addicts just aren't going to stop," he said. "And these people are the link to the heterosexual population."

As Hall and Moore proceed down Preston Street on their rounds, they greet many by name, passing men lined up alongside abandoned buildings, waiting to buy drugs. Both men are recovering addicts; Moore lives nearby with his wife and children.

One man walks out of a dilapidated row house, holding a ham sandwich, to ask them for condoms and bleach. Hall hands him a pamphlet and begins to tell him how to flush his syringe with bleach and water to kill any lingering virus. The man backs off. "Nah," he says, with a crooked smile, "I know how to do it."

Another man walks directly in front of Hall, who has been taking names of people who say they want to enter a drug treatment program. "I want to get rid of this," said a man who gave his name as Charles James, extending his hands, which were abscessed and swollen from years of heroin use. "I'll be there tomorrow."

Hall nods. As they talk, a teenager walks by and drops a small pink package of heroin on the street at James' feet. "Try this," he said. James picked it up and, in mid-sentence, turned abruptly and began following the youth down the street.

Hall, only slightly startled, turns to another man. A few minutes later, James returns, running. "Listen," he tells Hall earnestly, "I want you to know. I want to beat this."

Hall, Moore and Shellington say they have no way of knowing how many of the 24,000 people they encounter annually actually use condoms, rinse their needles or stop sharing their "works" -- the needle, cotton, syringe and spoon used to heat heroin before it can be injected.

"I don't even want to think I'm losing the battle," said Shellington. "I know this: If I'm helping one person, I'm not saving just one life. I'm saving the lives of how ever many contacts that person has."